1. Field of the Invention
The present invention relates to an electronic endoscope, and especially relates to the electronic endoscope having a connection structure between a coaxial cable and a solid-state image sensor.
2. Description Related to the Prior Art
An electronic endoscope has an insert section to be introduced into a human body cavity and an operation section for operating the insert section. The insert section is constituted of a distal portion, a bending portion, and a flexible portion arranged in this order from a distal end thereof. The distal portion is provided with an image capturing window, lighting windows, a medical instrument outlet, an airing/watering nozzle, and the like. Turning an angle knob on the operation section flexibly bends the bending portion from side to side and up and down to direct the distal portion in a desired direction inside the human body cavity.
The distal portion contains a solid-state image sensor that captures images of an internal body site through the image capturing window. To the solid-state image sensor, a printed circuit board is integrally attached. Signal cables connected to the printed circuit board extend through the insert section to the operation section. The insert section further contains angle wires for bending the bending section, a light guide for leading illumination light from a light source device to the lighting windows, and the like. An ultrasonic endoscope additionally has ultrasonic transducers provided at its distal portion and signal cables connected to the ultrasonic transducers.
As the signal cables connected to the printed circuit board, a cable bundle that consists of a plurality of bound coaxial cables is used. The coaxial cable consists of a center conductor positioned at the center thereof, insulation surrounding the center conductor, a braided wire surrounding the insulation, and an insulating jacket surrounding the braided wire. As described in Japanese Patent Laid-Open Publication No. 2001-95758, the center conductor is used as a signal line for transmitting an electrical signal between the solid-state image sensor and a processor device, and is joined to an input/output terminal of the printed circuit board by welding or the like. The braided wire is used as a ground line, and is joined to a ground terminal of the printed circuit board.
When the bending portion is bent into various directions in the human body cavity, the contents of the insert section shift in radial and axial directions of the insert section according to the bending. Thus, deformation stress applied to the cable bundle causes a break of a joint between the terminal of the printed circuit board and the coaxial cable.
The break of the joint occurs frequently in the center conductor used as the signal line. Since the braided wire in a net form is pulled out at an end of the insulating jacket of the coaxial cable, and is stranded into a single thick wire to be used as the ground line before being joined to the ground terminal, the braided wire has high strength and a firm solder joint. Since the center conductor, on the other hand, is just a single wire, the center conductor is inferior in strength to the braided wire and makes a weak solder joint. Japanese Patent Laid-Open Publication No. 2001-95758 neither describes a break of a joint nor takes measures thereto.